Ah, my dear Biblical-English-impaired friend, Acts 25:24 and Acts 26:27 both have "King Agrippa", while the rest (25:13, 25:26, 26:2, 26:7, 26:19) have "king Agrippa". Keenly observant fellows will notice that "King" in 25:24 is the beginning of a direct quote, therefore capitalized. "King" in 26:27 is the first word in the verse and therefore capitalized.
"Elementary, my dear Watson."